bg-image($url)
  background-image :url($url+"@2x.png")
  @media (-webkit-min-device-pixel-ratio:3),(min-device-pixel-ratio:3)
    background-image :url($url+"@3x.png")
line-1px($color)
  position :relative
  &:after
    display :block
    position :absolute
    left:0
    bottom :0
    border-top :1 px solid $color
    width :100%